I think that this bug is dependent on the order is which you load
various packgaes. If outline.el hasn't been parsed (and the defcustom
on line 57 hasn't been called) then the defvar in python-mode will set
outline-heading-end-regexp to nil which will eventually generate a
Lisp error: (wrong-type-argument stringp nil) in the bowels of
outline.el. If outline.el _has_ been evaluated (sorry, I don't know
all the correct lisp terms) before python-mode, then defvar is
harmless.

start with emacs -q

evaluate

(defvar outline-heading-end-regexp nil)
(require 'org-install)

in scratch

open any *.org file (that has at least 2 level of headers...I think
anything that will uses outline mode will work, but this is the one I
use)

expect: it to fold all but the top level headers without error
get: raises and error ( File mode specification error:
(wrong-type-argument stringp nil) ) and hides all the text

if you then evaluate

(setq outline-heading-end-regexp "\n")

[or set the value via customize] and re-load the .org file it behaves
as expected.